Stellar streams can also probe the clustering and distribution of dark matter at small scales.
The dark energy plus cold dark matter (\(\rm \Lambda CDM\) ) model predicts that dark matter should clump into gravitationally bound halos on scales that are much smaller than the smallest galaxies [1]}, [2]}, [3]}.
Dark matter subhalos that pass close to stellar streams may gravitationally perturb the stream by altering the stream track and inducing small-scale density fluctuations.
Discrete gaps in stellar streams, such as those found in the Pal 5 and GD-1 streams discovered from data collected by the Sloan Digital Sky Survey [4]}, [5]}, can probe the population of compact subhalos with \(10^6 \mathrm {\,M_\odot } < M < 10^8 \mathrm {\,M_\odot } \)  that contain no luminous matter [6]}, [7]}.
Additionally, the power spectrum of density fluctuations along a cold stream can place limits on the number of dark subhalos [8]} and the mass of warm dark matter candidates [9]}, [10]}. However, baryonic structures such as giant molecular clouds [11]} or Milky Way substructure such as the disk and bar [12]}, [13]}, [8]} can induce perturbations that mimic the observational signature of dark matter subhalos.
It is thus crucial to characterize cold streams at large Galactocentric radii where they are less likely to be affected by baryonic structures [15]}.
